Number of buttons

Physically separate buttons

For years this was the default type of touchpads: a touchpad with a
separate set of physical buttons below the touch surface. Such touchpads are
still around, but most newer models are Clickpads now.

Touchpads with physical buttons usually provide two buttons, left and right.
A few touchpads with three buttons exist, and Apple used to have touchpads
with a single physical buttons back in the PPC days. Touchpads with only two
buttons require the software stack to emulate a middle button. libinput does
this when both buttons are pressed simultaneously.

A two-button touchpad, with a two-button pointing stick above.

Note that many Lenovo laptops provide a pointing stick above the touchpad.
This pointing stick has a set of physical buttons just above the touchpad.
While many users use those as substitute touchpad buttons, they logically
belong to the pointing stick. The *40 and *50 series are an exception here,
the former had no physical buttons on the touchpad and required the top
section of the pad to emulate pointing stick buttons, the *50 series has
physical buttons but they are wired to the touchpads. The kernel re-routes
those buttons through the trackstick device.

Clickpads

Clickpads are the most common type of touchpads these days. A Clickpad
has no separate physical buttons, instead the touchpad itself is clickable
as a whole, i.e. a user presses down on the touch area and triggers a
physical click. Clickpads thus only provide a single button,
everything else needs to be software-emulated.

A clickpad on a Lenovo x220t. Just above the touchpad are the three buttons associated with the pointing stick. Faint markings on the bottom of the touchpad hint at where the software buttons should be.

Right and middle clicks are generated either via software
buttons or "clickfinger" behaviour. Software buttons define an area on the touchpad
that is a virtual right button. If a finger is in that area when the click
happens, the left button event is changed to a right button event. A middle click is
either a separate area or emulated when both the left and right virtual
buttons are pressed simultaneously.

When the software stack uses the clickfinger method, the number of fingers
decide the type of click: a one-finger is a left button, a two-finger click is a
right button, a three-finger click is a middle button. The location of the
fingers doesn't matter, though there are usually some limits in how the
fingers can be distributed (e.g. some implementations try to detect a thumb
at the bottom of the touchpad to avoid accidental two-finger clicks when the
user intends a thumb click).

The touchpad on a T440s has no physical buttons for the pointing stick. The marks on the top of the touchpad hint at the software button position for the pointing stick. Note that there are no markings at the bottom of the touchpad anymore.

Clickpads are labelled by the kernel with the INPUT_PROP_BUTTONPAD input property.

Forcepads

One step further down the touchpad evolution, Forcepads are Clickpads
without a physical button. They provide pressure and (at least in Apple's
case) have a vibration element that is software-controlled. Instead of the
satisfying click of a physical button, you instead get a buzz of happiness.
Which apparently feels the same as a click, judging by the reviews I've read
so far. A software-controlled click feel has some advantages, it
can be disabled for some gestures, modified for others, etc. I suspect that
over time Forcepads will become the main touchpad category, but that's a few
years away.

Not much to say on the implementation here. The kernel has some ForcePad
support but everything else is spotty.

Note how Apple's Clickpads have no markings whatsoever, Apple uses the clickfinger method by default.

Touch capabilities

Single-touch touchpads

In the beginning, there was the single-finger touchpad. This touchpad
would simply provide x/y coordinates for a single finger and get mightily
confused when more than one finger was present. These touchpads are now
fighting with dodos for exhibition space in museums, few of those are still
out in the wild.

Pure multi-touch touchpads

Pure multi-touch touchpads are those that can track, i.e. identify
the location of all fingers on the touchpad. Apple's touchpads support 16
touches (iirc), others support 5 touches like the Synaptics touchpads when
using SMBus.

Pure multi-touch touchpads are the easiest to support, we can rely on the
finger locations and use them for scrolling, gestures, etc. These touchpads
usually also provide extra information. In the case of the Apple touchpads
we get an ellipsis and the orientation of the ellipsis for each touch point.
Other touchpads provide a pressure value for each touch point. Though
pressure is a bit of a misnomer, pressure is usually directly related to
contact area.
Since our puny human fingers flatten out as the pressure on the
pad increases, the contact area increases and the firmware then calculates
that back into a (mostly rather arbitrary) pressure reading.

Because pressure is really contact area size, we can use it
to detect accidental palm contact or thumbs though it's fairly unreliable. A
light palm touch or a touch at the very edge of a touchpad will have a low
pressure reading simply because the palm is mostly next to the touchpad and
thus the contact area itself remains small.

Partial multi-touch touchpads

The vast majority of touchpads fall into this category. It's the half-way
point between single-touch and pure multi-touch. These devices can
track N fingers, but detect more than N. The current
Synaptics touchpads fall into that category when they're using the serial
protocol. Most touchpads that fall into this category can track two fingers
and detect up to four or five. So a typical three-finger interaction would
give you the location of two fingers and a separate value telling you that a
third finger is down.

The lack of finger location doesn't matter for some interactions (tapping,
three-finger click) but it can cause issues in some cases. For example, a
user may have a thumb resting on a touchpad while scrolling with two
fingers. Which touch locations you get depends on the order of the fingers
being set down, i.e. this may look like thumb + finger + third touch
somewhere (lucky!) or two fingers scrolling + third touch somewhere
(unlucky, this looks like a three-finger swipe).
So far we've mostly avoided having anything complex enough that requires the
exact location of more than two fingers, these pads are so prevalent that
any complex feature would exclude the majority of users.

Semi-mt touchpads

A sub-class of partial multi-touch touchpads. These touchpads can
technically detect two fingers but the location of both is limited to the
bounding box, i.e. the first touch is always the top-left one and the second
touch is the bottom-right one. Coordinates jump around as fingers move past
each other. Most semi-mt touchpads also have a lower resolution for two
touches than for one, so even things like two-finger scrolling can be very
jumpy.

Semi-mt are labelled by the kernel with the INPUT_PROP_SEMI_MT input property.

Physical properties

External touchpads

USB or Bluetooth touchpads not in a laptop chassis. Think the Apple Magic
Trackpad, the Logitech T650, etc. These are usually clickpads, the biggest
difference is that they can be removed or added at runtime. One interaction
method that is only possible on external touchpads is a thumb resting on the
very edge/immediately next to the touchpad. On the far edge, touchpads don't
always detect the finger location so clicking with a thumb barely touching
the edge makes it hard or impossible to figure out which software button
area the finger is on.

These touchpads also don't need palm detection - since they're not
located underneath the keyboard, accidental palm touches are a non-issue.

A Logitech T650 external touchpad. Note the thumb position, it is possible to click the touchpad without triggering a touch.

Circular touchpads

Yes, used to be a thing. Touchpad shaped in an ellipsis or circle.
Luckily for us they have gone full dodo. The X.Org synaptics driver had to
be aware of these touchpads to calculate the right distance for edge
scrolling - unsurprisingly an edge scroll motion on a circular touchpad
isn't very straight.

Graphics tablets

Touch-capable graphics tablets are effectively external touchpads, with two
differentiators: they are huge compared to normal touchpads and they have no
touchpad buttons whatsoever. This means they can either work like a
Forcepad, or rely on interaction methods that don't require buttons (like
tap-to-click). Since the physical device is shared with the pen input, some
touch arbitration is required to avoid touch input interfering when the pen
is in use.

Dedicated edge scroll area

Mostly on older touchpads before two-finger scrolling became the default method. These touchpads have a marking on the touch area that designates the edge to be used for scrolling. A finger movement in that edge zone should trigger vertical motions. Some touchpads have markers for a horizontal scroll area too at the bottom of the touchpad.